1. Field of the Invention
The present invention relates to an energy converter for converting energy into radiation of electromagnetic waves and also relates to a light source with such an energy converter.
2. Description of the Related Art
One of major obstacles that prevent an artificial light source from achieving high luminous efficacy is that the light source cannot convert energy into visible radiation without radiating a lot of infrared rays, of which the wavelengths are too long to sense with human eyes, at the expense of the visible radiation.
An incandescent lamp, used extensively today as a common illumination source, includes a filament functioning as a thermal radiator. The “thermal radiator” is a radiation source that emits an electromagnetic wave by thermal radiation. And the “thermal radiation” means radiation (of an electromagnetic wave) produced by applying heat energy to atoms or molecules of an object. The thermal radiation energy is determined by the temperature of the object and has a continuous spectrum. In the following description, the thermal radiator will be simply referred to herein as a “radiator”.
An incandescent lamp needs no ballasts, has a small size and a light weight, and shows a higher color rendering index than any other artificial light source. Due to these advantageous features, the incandescent lamp is a light source that is used most broadly worldwide.
To increase the radiation efficiency of incandescent lamps, people tried to raise the operating temperature of the radiator or to find a radiator that has a small radiation in the infrared range. History teaches us that a carbon filament as a radiator material for an incandescent lamp was replaced by the currently used tungsten filament as a result of those efforts. By using the radiator of tungsten, the radiator could operate at a higher temperature than the radiator of any other material and therefore could reduce the percentage of radiations in the infrared range.
However, in spite of their efforts, the radiation produced by current incandescent lamps, using the tungsten filament, in the visible wavelength range is just 10% of the overall radiations thereof. The majority of the other radiations are infrared radiations, which account for as much as 70% of the overall radiations. Also, the current incandescent lamps cause heat conduction due to an enclosed gap or a heat loss of 20% due to convection and have a luminous efficacy of about 15 lm/W, which is among the lowest ones in various artificial light sources. This performance of the incandescent lamps has not been improved significantly since 1930's.
Meanwhile, Japanese Patent Application Laid-Open Publication No. 03-102701 and other documents disclose a technique of drastically reducing the infrared radiations produced by a radiator and increasing the luminous efficacy of the lamp significantly. According to this technique, an array of very small cavities functioning as waveguides (which are termed “micro-cavities”) is provided on the surface of the radiator, thereby suppressing radiations of which the wavelengths exceed a predetermined value (e.g., infrared radiations) and selectively emitting only electromagnetic radiations with the predetermined wavelength. This patent document describes that cavities with a width of about 350 nm and a depth of about 7 μm are arranged at an interval of about 150 nm, thereby suppressing infrared radiations of which the wavelengths exceed about 700 nm. This patent document also describes that the luminous efficacy increases as much as six-fold at an operating temperature of 2,000 K to 2,100 K.
However, the micro-cavities disclosed in this patent document are tiny holes, of which the bottom is of a nanometer scale. Thus, it is not easy to make an array of such tiny micro-cavities on the surface of a filament.
Also, it was discovered that even when an array of micro-cavities with an inside diameter as small as 1 μm or less could be made on the surface of a filament made of tungsten or any other refractory material, those cavities collapsed during the operation. The present inventors discovered via experiments that such collapse occurred within a few minutes at 1,200 K, which is lower than the melting point of tungsten (of 3,650 K). Although Patent Document No. 1 is silent about the collapse of micro-cavities occurring at such a low temperature, this collapse would constitute a big obstacle to actually using a filament with those micro-cavities.
In order to overcome the problems described above, an object of the present invention is to provide an energy converter, of which the radiation suppressing portion for suppressing electromagnetic radiations with wavelengths exceeding a predetermined value has a sufficiently long life that has been extended so much as to use it actually, and also provide a light source including such an energy converter.